drm/radeon: Move AGP data structures into radeon

With the AGP code already duplicated, move over the AGP structures
from the legacy code base in to radeon. The AGP data structures that
are required by radeon are now declared within the driver. The AGP
instance is stored in struct radeon_device.agp.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210507185709.22797-3-tzimmermann@suse.de